Toxic heavy metals and nonferrous heavy metals are widely used in all industrial countries as components of alloys in steels and as finishing or corrosion-resistant coating materials in combination with plastics and natural products. In modern manufacturing technologies, metals from the A groups and some of the heavy metals from groups III-B, IV-B, and V-B of the periodic table provide advantages but also present environmental discharge problems.
By conservative estimates, about 5% of the metal elements produced or used in manufacturing or treatment processes are left behind as residues or wastes, in pure form or in compounds, which residues and wastes cannot be economically reused by presently known methods. In addition to the economic considerations, whereby it would be desirable to be able to economically recover the valuable materials and thus reduce the costs and foreign exchange requirements for purchasing the raw materials, from an environmental standpoint it is also important to be able to reduce the disposal burden of toxic and nonferrous heavy metals, which metals are toxic to nearly all biological species.
The removal of toxic and nonferrous heavy metals from waste waters of the metal processing industry is currently accomplished primarily by classical hydroxide precipitation with sodium hydroxide, milk of lime, or, in special cases, sodium carbonate. Because of the wide pH range in which the hydroxides and oxyhydrates precipitate, and because of mixed reactions, i.e., reactions of disparate divalent metal ions with different ion radii, for each metal ion mixture it is necessary to carry out preliminary tests to determine empirically the optimal "compromise" pH value. Further, it is not possible to make exactly reproducible predictions concerning the precipitation process, because numerous factors affect and can interfere with the precipitation process. Hydroxide precipitation of toxic and nonferrous heavy metals has basically four additional inherent disadvantages:
1. The solubility products of the metal hydroxides are at least 10.sup.7 -10.sup.10 greater than those of the corresponding sulfides. The solubilities of the metal hydroxides thus are cumulative, depending on the matrix conditions of the solutions, up to values tens of millions of times higher than those of the corresponding sulfides. PA1 2. The effect of neutral salts in the precipitation medium, which leads to increased solubility, is much stronger for hydroxide precipitation than for sulfide precipitation. Also, frequently neutral salts have deleterious effects on the sedimentation characteristics and the filterability of the precipitates. PA1 3. Hydroxide precipitation of some of the toxic and nonferrous heavy metals is impossible or inadequate in the presence of complex-forming agents. PA1 4. Metal hydroxides precipitated by classical methods cannot be reprocessed except by the use of difficult and costly methods. Therefore, as a rule, heretofore, hydroxide sludges from the metal processing industry have been disposed of at high cost as special toxic wastes. PA1 is highly water-soluble; PA1 causes precipitation of complexed and non-complexed metal ions, specifically toxic and non-ferrous heavy metal ions, and possibly organic or inorganic impurities, with very good separability of the precipitates; PA1 is usable over a very wide pH range; PA1 suppresses emission of hydrogen sulfide; and PA1 is very economical to manufacture. PA1 M represents an alkali or alkaline earth metal, PA1 x is 1 or 2, PA1 y is in the range 1.5x-2.5x, and PA1 z is in the range 0.1-2.5. PA1 In a preferred embodiment of the invention, PA1 M represents an alkali metal, particularly preferably sodium or potassium, more preferably sodium, PA1 x=2, PA1 y is in the range 1.5x-2.5x, particularly preferably 3.0-4.0, still more preferably 3.4-3.8, most preferably 3.6, and PA1 z is in the range 0.1-2.5, particularly preferably 0.3-1.5, still more preferably 0.5-1.0.
In contrast, the solubility products of most metal sulfides are so low that the metals can be precipitated quantitatively even from solutions containing strong complexes. Nonetheless, sulfide precipitation is rare in waste water treatment practice. One reason for this is the problems associated with the use of hydrogen sulfide, which is unpleasant smelling, toxic, and flammable; another reason is that most metal sulfides are difficult to remove to a satisfactory degree from the water phase.
In recent years, various organosulfides have been used in waste water purification practice. The organosulfides operate on the same principle as the sulfides, and precipitate, as sulfides, metals such as (among others) copper, cadmium, mercury, lead, nickel, tin, and zinc. However, organosulfides have the disadvantage that the allowable pH values are in the range &gt;7, because in the acid domain ineffective free acids are produced.
It happens that the stability constants of numerous heavy metal complexes, particularly those of the type of the frequently used polyaminocarboxylic acids, are strongly pH-dependent, such that the complexes are more stable at higher pH values than in the acid range. Thus the only way to precipitate complexed metals using conventional technology is to employ a complicated process involving re-complexing by Fe (III) ions.
In the complete absence of complexing substances, and by strict adherence to the optimal operating conditions, it is possible to meet the new maximum concentration values prescribed by the 40th General Administrative Regulations Specifying Minimum Requirements for Discharge of Waste Water into Publicly Regulated Waters, pursuant to .sctn. 7a of the German Water Management Law ("Wasserhaushaltsgesetz") with the use of hydroxide precipitation; however, a filtration step must be carried out following the precipitation. Such an overall process is costly, and does not assure constant adherence to the regulatory requirements, because the margin of safety between the achievable values and the regulatory limit values is only about 1.5-2.0. Therefore, quite small operating fluctuations lead to violation of the regulatory requirements.
In polysulfide precipitation of toxic and nonferrous heavy metals, one employs alkali and earth alkali polysulfides, instead of the toxic hydrogen sulfide. However, the polysulfide precipitating agents employed to date have had the disadvantage of low water-solubility, leading to high consumption of materials and high apparatus cost. Further, such agents are effective only with metal ions. There is a need for agents which can also remove, for example, pollutants of the mineral oil type.